メコン酸誘導体の研究 (第9報)

Ethyl 6-Hydroxycomenateの構造とそのFe³⁺による呈色について

抄録

Ethyl 6-hydroxycomenate (R) was obtained by bromination of comenic acid followed by hydrolysis and esterification but the shape of its ultraviolet spectrum, alkali titration curve, and formation of a metal chelate compound are different from those of other meconic acid derivatives. Moreover, its ultraviolet spectrum in aqueous solution is extremely labile and it was considered that the acid might have changed into 2-pyrone derivative during the course of this synthesis. However, the compounds formed by methylation of the two hydroxyls to O-methyl were negative both to coloration for β-diketone and to ferric chloride, and the coloration became positive only after ring cleavage by basification. Such evidences and the fact that methoxyacetone was formed by alkaline decomposition proved that the original compound was actually ethyl 5, 6-dihydroxy-4-pyrone-2-carboxylate. The extreme lability of ultraviolet spectrum of this compound in aqueous solution was considered to be due to its facile ring cleavage to form β-keto acid (cf. Chart 1). This compound (R) shows two kinds of coloration to ferric chloride, one of violet and the other of red, and this was found to be due to the strong reducing effect of the enediol, the mechanism of coloration to violet and red being respectively represented as shown in Charts 2 and 3.